Method for improving the oxidation stability of biodiesel as measured by the rancimat test

ABSTRACT

The oxidation stability of biodiesel fuel or mixtures of biodiesel fuel and conventional diesel fuel is improved by at least 50% by the addition to such fuel of at least 250 mg of N-M (C 1 -C 5 ) alkyl cyclohexyl amine per liter of the biodiesel fuel or per liter of the biodiesel fuel component in the mixture of biodiesel fuel and conventional diesel fuel as compared to the oxidation stability of said fuel without such additive.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to biodiesel fuels or mixtures ofbiodiesel fuel and conventional diesel fuel and to the improvement ofthe oxidation stability of such fuels by the use of additives.

2. Description of the Related Art

Biodiesel fuels which constitute the short chain alkyl esters ofvegetable or animal oils or fats, while having been identified in theart as useful for combustion in diesel engines or as blending stock formixtures with conventional diesel fuels are characterized as being lessstable than conventional diesel fuels.

Biodiesel fuels are mixtures of short chain alkyl esters of saturatedand unsaturated vegetable oils or animal oils or fats and as suchcontain olefinic double bonds as well as the ester moiety. Suchfunctionalities are susceptible to oxidative degradation leading to thedegradation of the biodiesel and its consequential unsuitability for usedue to its lack of long-term stability caused by the presence therein ofacids, alcohols, aldehydes, ketones, etc.

The improvement in the oxidation stability of biodiesel has been thesubject of investigation leading to the use of various additives andmixtures of additives to effect the desired stabilization.

WO 2008/056203 teaches stabilizer compositions for blends of petroleumand renewable fuels. Mixtures of renewable fuels such as biodiesel,ethanol and biomass mixed with conventional petroleum fuel arestabilized by the addition thereto of a multifunctional additive packagewhich is a combination of one or more additives selected from the groupconsisting of a free radical chain terminating agent, a peroxidedecomposition agent, an acid scavenger, a photochemical stabilizer, agum dispersant and a metal sequestering agent. Peroxide decompositionagents are selected from the group containing sulfur, nitrogen andphosphorus compounds. Suitable nitrogen-containing compounds are of thegeneral formula:

wherein R, R′ and R″ can be alkyl linear, branched, saturated orunsaturated C₁-C₃₀, aromatic, cyclic, poly alkoxy, polycyclic.Identified as a useful nitrogen containing compound isN,N-dimethylcyclohexylamine. While N,N-dimethylcyclohexylamine is taughtas a useful peroxide decomposition agent, in the examples it is neveremployed by itself but always in combination with a phenolicanti-oxidant. Reference to FIG. 2 of WO 2008/056203 reveals that whereasthe use of the combination of 75% phenol and 25%N,N-dimethylcyclohexylamine (at a treat level of 200 mg/l) resulted inan improvement in the relative stability of the fuel as compared tousing 100% phenol over all time periods tested, an increase in theamount of N,N-dimethylcyclohexylamine in the additive mixture to 50%significantly reduced the beneficial effect of the additive mixture(still at a treat level of 200 mg/l) in terms of relative stability overall time periods tested as compared to the 75% phenol/25%N,N-dimethylcyclohexylamine mixture with the most significant reductionin benefit being observed over the long term; i.e., at the six hour timeperiod.

US2004/0152930 teaches stable blended diesel fuel comprising an olefinicdiesel fuel blending stock containing olefins in an amount of 2 to 80 wt%, non-olefins in an amount of 20 to 98 wt % wherein the non-olefins aresubstantially comprised of paraffins, oxygenates in an amount of atleast 0.012 wt % and sulfur in an amount of less than 1 ppm, the blenddiesel being stabilized by an effective amount of a sulfur-freeantioxidant. An effective amount of sulfur-free antioxidant isidentified as 5 to 500 ppm, preferably 8 to 200 ppm of additive.

The sulfur-free antioxidant is selected from the group consisting ofphenols, cyclic amines and combinations thereof. Preferably the phenolscontain one hydroxyl group and are hindered phenols. The cyclic amineantioxidants are amines of the formula:

wherein A is a six-membered cycloalkyl or aryl ring, R¹, R², R³ and R⁴are independently H or alkyl and X is 1 or 2. An example of asulfur-free antioxidant is given as di-methylcyclohexylamine. See alsoU.S. Pat. No. 7,179,311.

“Evaluation of the Stability, Lubricity and Cold Flow Properties ofBiodiesel Fuel”, J. Andrew Waynick, 6^(th) International Conference onStability and Handling of Liquid Fuels, Vancouver, B.C., Canada, Oct.13-17, 1997, pages 805-829 addresses various aspects of biodiesel fueland reports an example where a blend of 80% low sulfur No. 2 dieselfuel/20% methyl soyate ester biodiesel fuel was combined with 20 ppmN,N-dimethylcyclohexylamine. At page 813 the report states that“although additive C (the N,N-dimethylcyclohexylamine) did not controlhydroperoxide or insolubles formulations, it did hold the TAN to a levelnear that of the fuel blend with anti-oxidant additive A(N—N,di-sec-butyl-p-phenylenediamine) and B (2,6-di-t-butyl-4-methylphenol)”.

U.S. 2008/0127550 discloses stabilized biodiesel fuel compositionwherein the stabilizing agent is a combination of: i) one or morecompounds selected from the group consisting of sterically-hinderedphenolic anti-oxidants; and ii) one or more compounds selected from thegroup consisting of triazole metal deactivators.

U.S. 2007/0289203 discloses a stabilized biodiesel wherein thestabilizing additive is a mixture of at least one aromatic diamine andat least one sterically-hindered phenol.

U.S. 2007/0151143 discloses a stabilized biodiesel wherein thestabilizing additive is selected from one or more of the groupconsisting of the 3-arylbenzofuranones and the hindered amine lightstabilizers and, optionally, one or more hindered phenolicanti-oxidants.

U.S. 2007/0248740 discloses an additive composition comprising2,5-di-tert-butyl hydroquinone (BHQ),N,N′-disalicylidenepropylenediamine. The additive is used to stabilizefuel containing at least 2% by weight of an oil derived from plant oranimal material.

U.S. 2007/0113467 discloses a biodiesel fuel composition having improvedoxidation stability. The fuel contains at least one anti-oxidant thatincreases the oxidative stability of the fuel selected from the grouprecited in paragraphs [0006] to [0012] of said published application.

U.S. Pat. No. 3,336,124 discloses stabilized distillate fuel oils andadditive compositions for such fuel oils. One additive compositioncomprises a mixture of: (a) an oil soluble dispersant terpolymer of aparticular type; (b) from 0.2 to about 3 parts by weight per part ofsaid oil soluble dispersant tripolymer of N,N-dimethylcyclohexylamine;and (c) a normally liquid inert hydrocarbon carrier solvent in an amountto constitute from about 20% to 80% by weight of the additivecomposition. See also GB 1,036,384.

DESCRIPTION OF THE INVENTION

The present invention relates to a method for improving the oxidationstability of biodiesel fuels) or mixtures or biodiesel fuel(s) andconventional diesel fuel(s) by the addition thereto of stabilityadditive selected from the group consisting of one or a mixture ofN,N-di(C₁-C₅-alkyl)cyclohexylamine, preferably one or a mixture ofN,N-di(C₁-C₁-alkyl)cyclohexylamine, most preferablyN,N-dimethylcyclohexylamine so as to increase the oxidation stability ofsaid additized fuel as measured by the Rancimat Test by at least about50%, preferably by at least about 60%, more preferably by at least about65%, most preferably by 100% or more as compared to the oxidationstability of the fuel without the stability additive, said methodcomprising adding to the biodiesel fuel or mixture of biodiesel fuel andconventional diesel fuel at least 250 mg, preferably at least 500 mg,more preferably at least 1000 mg of one or a mixture ofN,N-di(C₁-C₅-alkyl)cyclohexylamine(s), preferably one or a mixture ofN,N-di(C₁-C₂-alkyl)cyclohexylamine(s), mose preferablyN,N-dimethylcyclohexylamine per liter of the biodiesel fuel or per literof the biodiesel fuel component in the mixture of biodiesel fuel andconventional diesel fuel, preferably wherein one or a mixture ofN,N-di(C₁-C₅-alkyl)cyclohexylamine(s), preferably one or a mixture ofN,N-di(C₁-C₂-alkyl)cyclohexylamine(s), most preferablyN,N-dimethylcyclohexylamine is/are employed in the absence of anyphenolic and/or aminic anti-oxidant. For biodiesel fuels or mixture ofbiodiesel fuel and conventional diesel fuels which have oxidationstabilities of less than six hours, the addition to such fuels of theabove-described stability additive in the amounts indicated above canincrease the oxidation stability time to at least the six hour minimuminduction time necessary to satisfy the EN 14214 specification forstability of biodiesel fuels as measured by the Rancimat Test.

Biodiesel fuels are mixtures of lower, short chain esters of mixedsaturated and unsaturated straight chain fatty acids derived fromvegetable and/or animal fats and oils. The straight chain fatty acidsare, typically, C₁₀ to C₂₆ fatty acids, preferably C₁₂ to C₂₂ fattyacids. The fatty acids are made into biodiesel by transesterificationusing short chain alcohols; e.g., C₁ to C₅ alcohols, in the presence ofa catalyst such as a strong base.

Vegetable and/or animal oils and fats are natural triglycerides and arerenewable sources of starting material. Typical vegetable oils aresoybean oil, rapeseed oil, corn oil, jojoba oil, safflower oil,sunflower seed oil, hemp oil, coconut oil, cottonseed oil, sunfloweroil, palm oil, canola oil, peanut oil, mustard seed oil, olive oil,spent cooking oil, etc., without limitation. Animal fats and oilsinclude beef, pork, chicken fat, fish oil and oil recovered by therendering of animal tissue.

The biodiesel is made by esterifying one or a mixture of such oils andfats using one or a mixture of short chain; e.g., C₁ to C₅, alcohols,preferably methanol.

Transesterification is effected by the base catalyzed reaction of thefat and/or oil with the alcohol, direct acid catalyzed esterification ofthe oil and/or fat with the alcohol, conversion of the oil and/or fat tofatty acids and then to alkyl esters with as acid catalyst. In basecatalyzed transesterification, the oil and/or fat is reacted with ashort chain (such as methanol or ethanol) alcohol in the presence of acatalyst such as sodium hydroxide or potassium hydroxide to produceglycerin and short chain alkyl esters. The glycerin is separated fromthe product mixture and biodiesel is recovered. Any unreacted alcohol isremoved by distillation. The recovered biodiesel is washed to removeresidual catalyst or soap and dried.

In the present invention the fuel is either 100% biodiesel, typicallyidentified in the art as B 100, or it is a mixture of biodiesel withconventional diesel fuel; i.e., X % conventional diesel fuel and Y %biodiesel which would be identified as BY; for example, 80% conventionaldiesel and 20% biodiesel fuel would be described as B20.

As indicated above, in the present invention the biodiesel fuel has itsstability against oxidation increased by at least 50%, preferably by atleast 60%, more preferably by at least 65%, most preferably by 100% ormore, as measured by the Rancimat Test by the addition thereto of theabove-described stability additive in the amount indicated. Biodieselfuels or mixtures of biodiesel fuel and conventional fuels exhibitingoxidation stabilities of less than six hours can have their oxidationstabilities increased to an extent sufficient to meet the six hourinduction time specification established in EN 14214 for biodiesel fuelsas measured by the Rancimat Test (ISO 6886).

It has been found that the biodiesel can be combined withN,N-dimethylcyclohexylamine (DMCHA) in an amount of at least 250 mg,preferably at least 500 mg, more preferably at least 1000 mg of DMCHAper liter of biodiesel fuel (B100) or per liter of the biodiesel fuelcomponent in the mixture of biodiesel fuel and conventional diesel fuel.Expressed differently, one liter of B100 would contain at least 250 mgDMCHA while one liter of B20, for example, would contain 50 mg DMCHA,which is equal to at least 250 mg of DMCHA per liter of biodiesel fuelin the mixture of biodiesel fuel and conventional diesel fuel. Thus,because 5 liters of B20 would contain 1 liter of the transesterifiedbiodiesel fuel itself, 5 liters of B20 would be additized to contain atleast 250 mg of DMCHA.

The determination whether the fuel when additized has its oxidationstability increased by at least 50%, preferably by at least 60%, morepreferably by at least 65%, most preferably by 100% or more as comparedto the fuel without the additive, or, alternatively, meets the minimumsix hours induction time is made by subjecting the fuel to the RancimatTest (ISO 6886, per EN 14112). In the Rancimat Test, samples of liquidare aged at a constant temperature (110° C.) while air is passed throughthe liquid at a rate of 10 liters/hour. The exhaust airflow passesthrough a measuring cell filled with distilled water. The conductivityof the measuring cell is determined continuously and recordedautomatically. As the liquid oxidizes, volatile organic acids,aldehydes, ketones, alcohols, e.g., volatile oxygenates, are producedand taken up by the distilled water. This increases the conductivity ofthe water. The oxidation process is such that there is a gradualincrease in measured conductivity followed by a rapid increase. Theinflection point (not a specific value) of the conductivity curve is themeasured induction time. The length of the period prior to the rapidincrease, known as the “induction period”, is a measure of the oxidationstability of the liquid under test. The presence of an effectiveanti-oxidant will lengthen the induction period. The Rancimat Test hasbeen adopted as a specification test in the qualification of biodieselfuels.

The biodiesel fuels can contain other performance additives commonlyused in conventional diesel fuels such as dispersants, detergents,cetane improvers (such as 2-ethyl hexyl nitrate), demulsifiers,biocides, antifoaming agents, lubricity additives, dyes.

It has been found that to achieve the above-described increase inoxidation stability, it is not necessary to use any supplementalanti-oxidant (e.g., phenolic and/or aromatic amine) in addition to theDMCHA and, it is preferred that such supplemental phenol, preferablyhindered phenol and/or amine, preferably aromatic amine (e.g., diphenylamines or phenylnaphthyl amines) anti-oxidant not be employed in thestabilization process of the present invention.

EXAMPLES Description of the Evaluated Fuels

The Rancimat oxidation tests were run according to EN 14112. B100 canolamethyl ester (an unsaturated fatty acid methyl ester) was used as thebiodiesel fuel. The N,N-dimethylcyclohexylamine was acquired fromInnospec. Dicyclocarbodiimide acid scavenger Additin RC 8500 wasacquired from Akzo Nobel. The 2-ethylhexyl-3,4-epoxycyclohexanecarboxylate acid scavenger was acquired from ATOFINA.

-   Fuel 1: Base Biodiesel Fuel, B 100 Canola Methyl Ester (unsaturated    fatty acid methyl ester)-   Fuel 2: Fuel 1+500 mg/L N,N-dimethylyclohexyamine (DMCHA)-   Fuel 3: Fuel 1+1000 mg/L N,N-dimethylcyclohexylamine (DMCHA)-   Fuel 4: Fuel 1+500 mg/L Additin RC 8500 acid scavenger-   Fuel 5: Fuel 1+1000 mg/L Additin RC 8500 acid scavenger-   Fuel 6: Fuel 1+500 mg/L 2-ethylhexyl-3,4-epoxycyclohexane    carboxylate acid scavenger (EHEC)-   Fuel 7: Fuel 1+1000 mg/L 2-ethylhexyl-3,4-epoxycyclohexane    carboxylate acid scavenger (EHEC)

Example 1

This Example shows that N,N-dimethylcyclohexylamine is very effective isincreasing the Rancimat oxidation stability.

Rancimat Increase Induction Rancimat Time, Induction % Test FuelAdditive hours Time, hours Increase 1 Fuel 1 None 4.55 0 0% 2 Fuel 2DMCHA 7.46 2.91 +64% 3 Fuel 3 DMCHA 9.99 5.44 +119%

Example 2

It might have been thought that acid scavengers might retard Rancimatoxidation stability as light acid species are produced during oxidationprocess. This Example shows that acid scavengers were ineffective inimproving the Rancimat induction period.

Rancimat Increase Rancimat Induction Induction Time, Test Fuel AdditiveTime, hours hours 1 Fuel 1 None 4.55 0 4 Fuel 4 Additin RC 8500 4.570.02 5 Fuel 5 Additin RC 8500 4.56 0.01 6 Fuel 6 EHEC 4.52 −0.03 7 Fuel7 EHEC 4.62 0.07

1. A method for improving the oxidation stability of biodiesel fuel(s)or mixtures of biodiesel fuel(s) and conventional diesel fuel(s), saidmethod comprising the addition to such biodiesel fuel(s) or mixtures ofbiodiesel fuel(s) and conventional diesel fuel(s) a stability additiveselected from the group consisting of one or a mixture ofN,N-di(C₁-C₅-alkyl)cyclohexylamine(s) in an amount of at least 250 mg ofsaid one or a mixture of N,N-di(C₁-C₅-alkyl)cyclohexylamine per liter ofthe biodiesel fuel or per liter of the biodiesel fuel components in themixture of biodiesel fuel and conventional diesel fuel wherein theoxidation stability of said additized fuel as measured by the RancimatTest is increased by at least about 50% as compared to the oxidationstability of the fuel without the stability additive.
 2. The method ofclaim 1 wherein the stability additive is one or a mixture ofN,N-di(C₁-C₂-alkyl)cyclohexylamine(s).
 3. The method of claim 1 whereinthe stability additive is N,N-dimethylcyclohexylamine.
 4. The method ofclaim 1, 2 or 3 wherein the stability additive is added to the biodieselfuel(s) or mixture of biodiesel fuel(s) and conventional diesel fuel(s)in an amount of at least 500 mg of said stability additive per liter ofthe biodiesel fuel or per liter of the biodiesel fuel component in themixture of biodiesel fuel(s) and conventional diesel fuel(s).
 5. Themethod of claim 1, 2 or 3 wherein the stability additive is added to thebiodiesel fuel(s) or mixture of biodiesel fuel(s) and conventionaldiesel fuel(s) in an amount of at least 1000 mg of said stabilityadditive per liter of the biodiesel fuel(s) or per liter of thebiodiesel fuel(s) component in the mixture of biodiesel fuel(s) andconventional diesel fuel(s).
 6. The method of claim 1, 2 or 3 whereinthe oxidation stability is increased by at least about 65%.
 7. Themethod of claim 4 wherein the oxidation stability is increased by atleast about 65%.
 8. The method of claim 5 wherein the oxidationstability is increased by at least about 65%.
 9. The method of claim 1,2 or 3 wherein the oxidation stability is increased by 100% or more. 10.The method of claim 4 wherein the oxidation stability is increased by100% or more.
 11. The method of claim 5 wherein the oxidation stabilityis increased by 100% or more.
 12. The method of claim 1, 2 or 3 whereinthe stability additive is employed in the absence of any phenolic and/oraromatic amine antioxidant.
 13. The method of claim 4 wherein thestability additive is employed in the absence of any phenolic and/oraromatic amine antioxidant.
 14. The method of claim 5 wherein thestability additive is employed in the absence of any phenolic and/oraromatic amine antioxidant.
 15. The method of claim 6 wherein thestability additive is employed in the absence of any phenolic and/oraromatic amine antioxidant.
 16. The method of claim 7 wherein thestability additive is employed in the absence of any phenolic and/oraromatic amine antioxidant.
 17. The method of claim 8 wherein thestability additive is employed in the absence of any phenolic and/oraromatic amine antioxidant.
 18. The method of claim 9 wherein thestability additive is employed in the absence of any phenolic and/oraromatic amine antioxidant.
 19. The method of claim 10 wherein thestability additive is employed in the absence of any phenolic and/oraromatic amine antioxidant.
 20. The method of claim 11 wherein thestability additive is employed in the absence of any phenolic and/oraromatic amine antioxidant.